When you walk away from your Mac, the screen turns off and the system locks. If you receive a notification—such as a direct message on Slack, a text via iMessage, or a calendar reminder—the lock screen will briefly illuminate and display the notification. By default, macOS automatically shows the content (the preview) of that notification directly on the lock screen.
This is a massive privacy vulnerability in a shared environment like a university library or an open-plan office. If a coworker or spouse sends you a highly sensitive, confidential message, anyone casually walking past your locked Mac will be able to read the entire first sentence of the message right off the screen without needing your password.
You can configure macOS to alert you that a notification has arrived, while completely hiding the sensitive text until you authenticate via Touch ID or your password.
How to Hide Notification Previews on the Lock Screen
You can change this global privacy setting in the Notifications menu.
- Open the System Settings (or System Preferences) via the Apple Logo in the top-left menu bar.
- Scroll down the left sidebar and click on Notifications.
- At the very top of the right pane, look for the drop-down menu labeled Show previews:.
- Click the drop-down menu and change the selection from “Always” to When unlocked. (You can also choose “Never” if you don’t even want previews when actively using the computer).
The change is immediate. If you receive an iMessage while the Mac is locked, the screen will light up and simply say “Messages: 1 Notification,” completely concealing the identity of the sender and the content of the text until you unlock the machine.